Antarctic Space Science - Publications
2005
[back]Pc 1 waves and associated unstable distributions of magnetospheric protons observed during a solar wind pressure pulse
Authors: Arnoldy, R. L.; Engebretson, M. J.; Denton, R. E.; Posch, J. L.; Lessard, M. R.; Maynard, N. C.; Ober, D. M.; Farrugia, C. J.; Russell, C. T.; Scudder, J. D.; Torbert, R. B.; Chen, S.-H.; Moore, T. E.
Journal: Journal of Geophysical Research, Volume 110, Issue A7, CiteID A07229
Date: Jul 2005
Abstract: We present observations of Pc 1 waves (~0.6 Hz) that occurred shortly after a strong (>20 nPa) compression of Earth's magnetosphere at 1321 UT, 18 March 2002. Intense Pc 1 waves were observed at several high-latitude ground stations in Antarctica and Greenland from 1321 UT to beyond 1445 UT. Two wave bursts were recorded at the Polar satellite at 1338 and 1343-1344 UT as it passed outbound in the Southern Hemisphere at 1154 local time (SM magnetic latitude of -22° and near L = 7.5) in good magnetic conjunction with the Antarctic. The pressure increase created a significant population of protons between a few hundred eV and several keV, whose fluxes were mostly perpendicular to B. These protons seem to have replaced the quiescent stream of protons (presumably convected from the plasma sheet) that existed before this increase. There was also a nearly two-order-of-magnitude increase in the population of thermal/suprathermal (0.32-410 eV) protons. The generation of ion cyclotron waves is expected to limit the proton temperature anisotropy A, defined as T$\perp$/T$\parallel$ - 1. The ion cyclotron instability driven by the observed hot ion temperature anisotropy is studied using two models, with and without the presence of cold background plasma. Peaks in the calculated instability as a function of time show excellent agreement with the times of the Polar wave bursts, which were measured a few tens of seconds after maxima in the instability calculation. The time delay is consistent with the propagation time to the spacecraft from a source nearer to the equatorial plane. The hot proton population at Polar appears to be driven back to stability by a sudden increase in very field-aligned protons having energies less than the hot perpendicular population, suggesting a different source for the two populations. These observations confirm the importance of both the energization and/or increase in population of protons transverse to B in the several keV range (possibly betatron acceleration as a result of the pressure pulse), and the presence of greatly increased fluxes of lower energy protons (100s of eV to a few keV), predominantly aligned along B, in determining whether the particle population is unstable at a given time.
On the source of Pc1-2 waves in the plasma mantle
Authors: Engebretson, M. J.; Onsager, T. G.; Rowland, D. E.; Denton, R. E.; Posch, J. L.; Russell, C. T.; Chi, P. J.; Arnoldy, R. L.; Anderson, B. J.; Fukunishi, H.
Journal: Journal of Geophysical Research, Volume 110, Issue A6, CiteID A06201
Date: Jun 2005
Abstract: Several ground-based observational studies have reported unstructured Pc1-2 waves near and poleward of the dayside cusp. Dyrud et al. (1997) were the first to suggest that these waves originated in the high-altitude plasma mantle. We report here on simultaneous field and particle observations from the Polar satellite and from ground magnetometer data that confirm the presence of these ``mantle'' waves and characterize the ion distributions associated with them. Unstructured Pc1-2 waves were found to occur during 45% of the available passes of Polar across the mantle during the interval March-May 2000, during which time the orbit of Polar was near local noon as it crossed these latitudes. Poynting vector calculations using Polar electric and magnetic field data show a mixture of upward and downward energy flux in the cusp but consistently downward flux when mantle waves are observed. This is consistent with earlier observations that ground-based magnetometers can detect such mantle waves but not the more intense wave activity observed by spacecraft in the middle- and low-altitude cusp. In situ particle observations from the Hydra instrument appear to confirm the generation mechanism suggested by Dyrud et al. that cusp particles mirroring at low altitudes and then traveling outward in the plasma mantle poleward of the cusp are the source of the free energy for these waves. Quantitative estimates of ion cyclotron instability of the observed ion distributions suggest they are stable at the <=9 RE altitudes traversed by Polar but may be reasonably inferred to be unstable in the exterior mantle, a location consistent with their observed frequency.
Wave acceleration of electrons in the Van Allen radiation belts
Authors: Horne, Richard B.; Thorne, Richard M.; Shprits, Yuri Y.; Meredith, Nigel P.; Glauert, Sarah A.; Smith, Andy J.; Kanekal, Shrikanth G.; Baker, Daniel N.; Engebretson, Mark J.; Posch, Jennifer L.; Spasojevic, Maria; Inan, Umran S.; Pickett, Jolene S.; Decreau, Pierrette M. E.
Journal: Nature, Volume 437, Issue 7056, pp. 227-230 (2005).
Date: Sep 2005
Abstract: The Van Allen radiation belts are two regions encircling the Earth in which energetic charged particles are trapped inside the Earth's magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.
Density enhancement in plasmasphere-ionosphere plasma during the 2003 Halloween Superstorm: Observations along the 330th magnetic meridian in North America
Authors: Chi, P. J.; Russell, C. T.; Foster, J. C.; Moldwin, M. B.; Engebretson, M. J.; Mann, I. R.
Journal: Geophysical Research Letters, Volume 32, Issue 3, CiteID L03S07
Date: Jan 2005
Abstract: On October 29-31, 2003, the ground observations of field line resonance signals and the total electron content (TEC) along the 330th magnetic meridian recorded extraordinary density variations in both the magnetosphere and the ionosphere. In the magnetosphere, the density decreased at outer L shells due to strong convection, whereas it increased significantly in the afternoon sector at L <= 4. In the ionosphere, a strong positive storm occurred at low latitudes, and storm enhanced density was also observed at approximately 1400 LT in mid-latitude regions. The density enhancements in both the magnetosphere and ionosphere coincided with intervals of southward IMF and high-speed solar wind, consistent with the scenario that the eastward electric field imposed on the ionosphere led to a positive storm which might contribute to the dense plasmaspheric drainage plume. These results demonstrate that the ionosphere can be an important factor modulating the density variations in a storm-time plasmasphere.
Statistical studies of auroral MF burst emissions observed at South Pole Station and at multiple sites in northern Canada
Authors: LaBelle, J.; Weatherwax, A. T.; Tantiwiwat, M.; Jackson, E.; Linder, J.
Journal: Journal of Geophysical Research, Volume 110, Issue A2, CiteID A02305
Date: Feb 2005
Abstract: Auroral MF burst is a broadband impulsive natural radio emission of auroral origin observed at ground level in the frequency range 0.8-4.5 MHz. Application of a semiautomated analysis method to data collected at South Pole Station, 74° magnetic latitude, results in a database of frequencies, amplitudes, and universal times of MF burst emissions observed during 1 January-31 December 2003. Besides confirming that MF burst emissions are observed at ground level only during darkness and that magnetic local time plays a significant role in controlling the MF burst occurrence rate, which maximizes in the pre-magnetic-midnight hours, these data reveal that the frequencies of MF burst emissions tend to be higher just after sunset and before sunrise than they are during midwinter. This observation supports a link between the emission frequency and the maximum plasma frequency in the ionosphere. South Pole data also show evidence for MF burst extending to frequencies well below the ionospheric gyrofrequency, which brings up the possibility that they are partly in the whistler mode. Analysis of optical and riometer data from one extra-low-frequency MF burst example suggests that MF burst is associated with a relatively high-energy (>10 keV) auroral electron beam. Data from multiple Canadian observatories during 1997-1998 show that the MF burst occurrence rate maximizes near 74° magnetic latitude. With observatories spaced by 260-585 km, about 10% of the MF burst events are observed at two or more stations. Among coincident events, about 3/4 occur first at the lower-latitude station and shift to the higher-latitude station. The average duration of the MF burst events detected at two or more observatories increases with latitude.
Alfven wave reflection in a curvilinear magnetic field and formation of Alfvenic resonators on open field lines
Authors: Pilipenko, V. A.; Mazur, N. G.; Fedorov, E. N.; Engebretson, M. J.; Murr, D. L.
Journal: Journal of Geophysical Research, Volume 110, Issue A10, CiteID A10S05
Date: Jun 2005
Abstract: Though shear Alfven waves have no reflection points, these waves can be partially reflected from sharp field-aligned variations in Alfven velocity. Such a reflection can lead to the formation of quasi-resonators, such as the ionospheric Alfven resonator, in which wave energy can be accumulated. We show that a similar partial reflection of Alfven waves can occur in regions with steep variations of geomagnetic field line geometry. For that, the propagation of Alfven waves in a plasma immersed in a two-dimensional (2-D) curvilinear magnetic field has been investigated. The Alfven waves comprise two modes with different polarization, toroidal and poloidal, which are described by 1-D equations. These equations formally coincide with the equation for the case of a quasi-uniform straight magnetic field with a modified Alfven velocity that takes into account the field-aligned dependence of the Lamé coefficients. The toroidal and poloidal modes depend differently on the magnetic field geometry. The poloidal mode is efficiently reflected from regions where the magnetic field lines sharply converge or diverge, when its wavelength becomes comparable to the typical scale of magnetic field convergence. This effect can result in the formation of Alfven quasi-resonators on open field lines. This mechanism can be used to interpret the occurrence of specific polar cap quasi-periodic Pi3 pulsations with periods ~15-20 min observed by the AGO magnetometer array in Antarctica. Also, such a reflection can limit the influx of Alfvenic wave energy along reconnected field lines from the solar wind into the magnetosphere at certain wavelengths. This effect has been observed as the occurrence of a low-pass cutoff period ~10-15 min in the spectral transfer function between conjugate IMF variations and ground magnetic disturbances.